1. Technical Field
The present invention relates to a printed wiring or circuit board.
2. Description of the Prior Art
In general, a printed wiring board is formed with a necessary circuit pattern, on an insulated base plate, of electric conductor, and necessary electronic parts are assembled in the circuit pattern.
Assembling of the electronic parts is carried out such that, after the electronic parts are mounted on the printed wiring board, the circuit pattern surface of the printed wiring board is dipped in a melted solder bath or a jet stream solder bath to apply the solder to the circuit pattern surface to electrically and mechanically connect the circuit pattern with leads of the electronic parts.
To perform the soldering of the circuit pattern with the leads of the electronic parts, previously all the surface of the circuit pattern is covered by a solder-resist film except the land area to be soldered, to feed solder only to the necessary portion on the circuit pattern. As the circuit pattern is smaller, a distance between adjacent circuits is also narrower and distance between connection lands is narrower, to result in bridging between the lands by soldering, to impair the function of the solder-resist film. This results in repair operation afterwards.
To prevent the bridging of the solder, especially to prevent the bridging of solder between narrow distance land portions, an improved printed wiring board manufacturing process is disclosed such that, an electrically conductive pattern is printed on the insulated base plate, all of the surface of the conductive pattern is covered by a first layer of solder resist film except land portions to be soldered, and a second layer of bridge preventing solder resist film is covered on the first layer at least between narrow distance land portions to prevent bridging of solder. The process is disclosed in Japanese Patent Publication No. 41162/1979.
However, the process must be performed by forming the first solder resist layer, and forming the second bridge preventing solder resist layer. As the first resist layer covers all the surface of the conductive pattern except land portions to be soldered, highly accurate alignment is required to expose the land portions to be soldered. Further as the second bridge preventing solder resist layer is formed on the first solder resist layer after the first solder resist layer is formed, another highly accurate alignment is required, and also solder resist ink of screen printing type used to form solder resist layer tends to spread over the land portions to be soldered.
Accordingly, the applicant of the present invention filed Japanese Patent Application No. 37647/1987 in which is disclosed an improved printed wiring board effective to mitigate the above mentioned disadvantages of the known printed wiring board, the improved printed wiring board comprised of a solder-resist layer having necessary solder resist function without adding any complicated technical operations to the conventional solder resist covering operation.
More particularly, the solder resist film on the printed wiring board can positively prevent flux and solder from adhering on the surface due to characteristics of silicone and/or fluorine series resin contained in the film. Further, when the film is formed by solder resist printing ink on another surface of the base plate having a circuit pattern one one surface, invasion of flux or solder through through-holes or parts insert holes formed in the printed wiring board is also positively prevented.
FIG. 1 is an enlarged sectional view of a portion of the printed wiring board having the above described solder resist film. In the drawing, 1 designates an insulated base plate, 2 is a circuit patern formed of electrically conductive copper film on one surface 1a of the insulated base plate, 3 is a connection land in the circuit pattern 2, 4 is a through hole open at the connection land 3, 6 is a solder resist film coated on the surface of the circuit patern 2, and 7 is another solder resist film coated on the other surface 1b of the insulated base plate 1.
The solder resist film 6 covers all the surface of the circuit patern 2 except the connection land 3, and is formed 7 by means of, for example, silk screen printing process utilizing printing ink containing insulating silicone or fluorine series resin which can prevent flux or solder from adhering on the surface.
Examples of composition of the solder resist printing ink which is used to form the solder resist films 6 and 7 are as follows:
______________________________________ Composition 1 epoxy acrylate 28 wt. part polyethylene glycol acrylate 72 benzoin alkylether 4 titanium dioxide 5 silicon dioxide 3 diphenyl disulfide 2.0 pigment (cyaning green) 0.4 diethyl hydroxy amin 0.1 dimetyl siloxane (anti-foam agent) 2.0 silicone series high polymer resin 2-5 Composition 2 epoxy acrylate 28 wt. part polyethylene glycol acrylate 72 benzoin alkylether 4 titanium dioxide 5 silicon dioxide 3 diphenyl disulfide 2.0 pigment (cyaning green) 0.4 diethyl hydroxy amin 0.1 dimetyl siloxane (anti-foam agent) 2.0 fluorine series surface active agent 2-5 (Florard FC-170C Sumitomo 3-M Co.) Composition 3 epoxy acrylate 50 wt. part polyuretane acrylate 50 benzoin methylether 4 calsium carbonate 5 silicon dioxide 3 cyaning green 0.4 benzo thiazole 0.05 benzo phenon 2.6 dimethl siloxane 1.5 silicon series high molecule resin 2-5 Composition 4 epoxy acrylate 50 wt. part polyuretane acrylate 50 benzoin methylether 4 calsium carbonate 5 silicon dioxide 3 cyaning green 0.4 benzo thiazole 0.05 benzo phenon 2.6 dimethl siloxane 1.5 silicon series high molecule resin 2-5 fluorine series surface active agent 2-5 (Florard FC-430, Sumitomo 3-M Co.) ______________________________________
In the above mentioned compositions, substantial market-available examples of the silicon series and fluorine series resins are follows: